1. Field of the Invention
The present invention relates to a surface protective film that is provided with a film-shaped support and a pressure-sensitive adhesive (PSA) provided on the support, and that protects an adherend by being laminated to the adherend.
2. Description of the Related Art
Surface protective films (to also be referred to as surface protective sheets) typically have a configuration in which a PSA is provided on a film-shaped support (substrate). These protective films are laminated to an adherend by means of a PSA as described above, and are therefore used for the purpose of protecting the adherend from damage and soiling during processing or transport. For example, liquid crystal display panels are formed by laminating optical members such as a polarizing plate or retardation plate to liquid crystal cells by means of PSA. Protective films are laminated to these optical members laminated to the liquid crystal cells by means of PSA for the purpose of preventing damage, soiling and the like. These protective films are then removed by being peeled from the optical members (adherends) together with the PSA at the stage the protective films are no longer required such as when the optical members are laminated to the liquid crystal cells. Examples of technical literatures relating to surface protective films include Japanese Patent Application Publication Nos. 2008-69261 and 2005-23143. Japanese Patent Application Publication No. H11-116927 relates to a technology for improving moisture permeability of a PSA layer.
The peel strength when peeling a protective film adhered to an adherend from the adherend can vary according to, for example, the surface state and the materials that compose the surface of the adherend. A detailed explanation of this is provided using the example of the case of the adherend being a polarizing plate. Normally, a polarizing plate has a structure in which a protective layer composed of triacetyl cellulose (TAC) is laminated to a polarizer composed of polyvinyl alcohol (PVA) by means of PSA. This protective layer is imparted with functions corresponding to its application. For example, anti-glare (AG) treatment is carried out on the surface of the protective layer by forming fine surface irregularities therein in order to reduce glare caused by ambient light. As a result, in contrast to the surface (TAC side) of a polarizing plate not imparted with such a function (plain polarizing plate) being smooth, the surface (AG side) of a polarizing plate subjected to AG treatment (AG polarizing plate) has surface irregularities. The peel strength of the protective film tends to differ greatly between the TAC side and the AG side depending on such differences in surface state (surface structure). Moreover, this AG treatment is typically by coating (AG coating) the surface of a TAC protective layer using a coating agent composed of a material having lower polarity than TAC. As a result, in contrast to the TAC side being smooth and having comparatively high polarity, the side on which the AG coating is carried out (AG side) has surface irregularities and relatively low polarity. These differences in chemical characteristics and differences in physical structure result in large differences in the peel strengths of the TAC side and AG side. This difference in peel strength can cause a decrease in work efficiency when peeling the protective film from the polarizing plate. However, costs are incurred in order to make peel strength uniform by using different protective films corresponding to surface state.
Japanese Patent Application Publication No. 2008-69261 describes that differences in adhesiveness can be reduced according to the surface state of an adherend and the like by a PSA composition containing a (meth)acrylic polymer, obtained by copolymerizing a monomer having an alkylene oxide structure, and an ionic liquid salt. However, this technology demonstrates this effect by having a ionic liquid salt as an essential constituent thereof (paragraphs 0061 to 0062 of this publication), and in actuality, according to studies conducted by the present inventor, a PSA composition in which the ionic liquid salt has been excluded from the composition of Japanese Patent Application Publication No. 2008-69261 was confirmed to not allow the obtaining of the effect of reducing differences in adhesiveness attributable to differences in surface state. In order to realize soiling resistance to an adherend at a high level, it is desirable to avoid a composition that essentially requires the use of an ionic liquid salt. Japanese Patent Application Publication No. 2005-23143 relates to a technology for reducing dependency of peel strength on peeling speed by lowering high-speed peeling strength, not for reducing differences in peel strength caused by differences in surface state of an adherend.
In addition, protective films are required to have a property that allows it to tightly (closely) adhere to an adherend to which the protective film is adhered (adhesiveness). If a protective film is in a state in which it has lifted from the surface of the adherend, the appearance of the adherend laminated to the protective film changes due to air bubbles present between the protective film and the adherend in cases in which the adherend is subjected to external force or temperature changes. As a result, a difference in appearance occurs between a portion subjected to this external force and the like and a portion not subjected thereto, and this difference in appearance causes a decrease in the accuracy of visual inspections, and may even cause a decrease in yield.
Since fine surface irregularities are formed in the AG side of a polarizing plate as previously described, a protective film provided with a PSA that easily deforms along the surface irregularities (easily fills in the surface irregularities) is advantageous for tightly adhering the protective film to the AG side. However, when adhesiveness to a surface having fine surface irregularities (such as the AG side) is attempted to be improved, the difference in peel strength between the irregular surface and the flat surface generally tends to become even greater. Namely, improvement of adhesiveness to an adherend and reduction of differences in peel strength caused by differences in the surface state of an adherend are in a relationship such that the first characteristic decreases when the second characteristic is improved, thereby creating the need to enable each characteristic to be realized simultaneously.